ISA versus SIPP wealth building: how the maths stacks up — with Greggs as the test case

The ISA versus SIPP wealth building debate has a clear structural answer when you run the arithmetic, even if the right choice for any individual depends on when they plan to draw the money down. Both wrappers can hold the same shares, but they treat your contributions and your withdrawals very differently.

Why a SIPP pulls ahead on the accumulation side

The key advantage of a Self-Invested Personal Pension (SIPP) is tax relief on contributions. A Stocks and Shares ISA offers none: every £1 you put in is exactly £1 invested. With a SIPP, basic-rate taxpayers effectively invest £1.25 for every £1 contributed, because the government tops up the pension with the 20% income tax you already paid. Higher and additional rate taxpayers can claim back a further slice through their tax returns, widening that gap further.

The second structural advantage is the annual allowance. A Stocks and Shares ISA caps contributions at £20,000 per tax year. The SIPP allowance sits at £60,000. For anyone at or near the top of their earnings capacity, that extra headroom matters over a compounding time horizon. All else equal, the same investment decisions made inside a SIPP generate more wealth than they would inside an ISA, simply because more capital is working from day one.

Please note that tax treatment depends on the individual circumstances of each client and may be subject to change in the future. The content here is provided for information purposes only and does not constitute tax advice.

The ISA’s case: flexibility that a SIPP cannot match

The SIPP’s edge on accumulation comes with a binding constraint: you cannot access the money before age 55, a threshold set to rise to 57. An ISA can be drawn on at any point. Dividends and capital gains are sheltered from tax inside both wrappers, but when you start taking money out the gap opens again. ISA withdrawals are entirely tax-free. SIPP withdrawals carry a 25% tax-free allowance; beyond that, withdrawals are treated as income and potentially subject to income tax. Holding both wrappers in parallel, as many investors do, allows you to manage taxable income in retirement by choosing which pot to draw from and when.

Greggs as an ISA versus SIPP equity holding

The structural question of which wrapper to use sits alongside the more immediate question of what to hold inside them. One stock that illustrates the trade-offs between income, growth and near-term cost pressure is Greggs (LSE: GRG), the bakery chain.

The shares offer a yield of 4.4% at current prices, but the full-year 2025 numbers tell a mixed story. According to Greggs’ 2025 preliminary results, total group sales grew 6.8% to £2,151 million, but underlying operating profit fell 4.0% to £187.5 million and underlying profit before tax dropped 9.4% to £171.9 million. Underlying diluted earnings per share came in at 122.8p, down from 137.5p the year before. The underlying return on capital employed fell from 20.3% to 16.0%.

The estate expansion that has driven sales growth is also the main drag on margins. As of 28 December 2024, Greggs operated 2,618 shops, having opened an average of four new locations every week through 2024. The longer-term ambition, set out in the company’s five-year plan launched in 2021, is an estate of more than 3,000 shops. New national distribution centres in Derby and Kettering, described in the 2025 annual report, are designed to support an estate of up to 3,500 stores.

The cost of that infrastructure is visible in the numbers. On top of higher capital expenditure, National Insurance increases and wage pressure have weighed on margins, and conflict in the Middle East poses a further risk to ingredient and energy costs.

The more recent data argues for some patience. Interim results for the 26 weeks ended 27 June 2026 showed total sales up 7.2% year on year to £1.10 billion and operating profit rising nearly 23% to £86.5 million. Like-for-like sales in company-managed shops grew 1.6% in the first nine weeks of 2026. Management expects profit before tax for the full year 2026 to be broadly in line with 2025, which sets the bar low enough that a modest improvement in the second half would read positively against consensus.

The share price has fallen by roughly two-fifths over five years, reflecting those margin concerns. Whether the current yield and the longer-term estate optionality are adequate compensation for the near-term profit headwinds is the question investors need to answer before sizing a position in either wrapper. For anyone running a SIPP with a decade-plus horizon, the compounding effect of the 25% tax uplift on contributions makes the case for holding on through the expansion phase more compelling than it would appear in an ISA on a shorter time frame. The next read on the thesis comes with the full-year 2026 results.